The invention disclosed and claimed herein is related to high energy linear charged particle accelerators of the kind used to accelerate protons, electrons or ions.
Linear particle accelerators are used to produce beams of electrically charged nuclear or atomic particles. Low energy linear particle accelerators include cathode ray tubes, x-ray generators, and other similar devices. High energy linear accelerators, known as linacs, are larger and more complex, typically ranging from approximately one meter to several kilometers in length.
Linear accelerators are used in medicine for radiotherapy purposes and in industry as testing electron accelerators and for other purposes. They are also used in high energy nuclear physics research. Proton accelerators, for example, are used as drivers for neutrino experiments and as spallation neutron sources, and are of potential use in driving and controlling sub-critical nuclear reactors. Another potential use of high energy accelerators is the transmutation of radioactive nuclear waste to benign nonradioactive elements.
A standing wave linear accelerator typically includes a series of resonant cavities positioned along a longitudinal axis that defines a beam line, which is the path of travel of the accelerated particles. The cavities are connected by beam tube segments, which may be integral with the cavities and which form a beam tube that opens into each cavity. The cavities and the beam tube segments are electrically conductive, generally being constructed of copper; and the entire beam line, including the beam tube segments and the cavities, is evacuated.
The cavities are coupled to a power source that introduces a radio frequency (RF) power signal into the cavities, typically a klystron that produces a power signal in the microwave frequency range, to establish and maintain a standing wave in the cavities. The standing microwave signal provides the alternating electrical fields that accelerate charged particles as they pass through each cavity.
As charged particles pass through the successive cavities along the beam line, some or all of the cavities provide additional acceleration of the particles. The particles are typically bunched so that they arrive at the accelerating cavities in phase with the sinusoidally varying electric fields in the cavities. The beam tube segments connecting the cavities act as a Faraday cage, such that no acceleration occurs within the beam tube segments. The combined acceleration of all of the cavities along the beam line results in the particles being accelerated to their maximum velocity and energy as they are emitted from the accelerator, which velocity may approach but not exceed the velocity of light The ratio of the velocity of an accelerated particle to the velocity of light is generally represented as β, where β=v/c, and in many applications a goal in designing an accelerator is to attain the highest value of β as is feasible, given design and cost constraints.
Acceleration within a cavity is caused by the force of the resonating electric field component of the standing wave acting on the particles as they pass through the cavity. In order to achieve optimum acceleration of a particular kind of particle passing along the beam line, the sizes and shapes of the cavities, the spacing between cavities, and the phases of the resonant signals within the cavities at each point along the beam line, must all be selected so that the direction and amplitude of the resonating electric field in each cavity are timed to achieve maximum forward acceleration of the charged particles as they pass through the cavity. In this regard, successive cavities along the beam line are typically spaced apart by increasingly greater distances toward the emission end of the accelerator, such that the distance between any two adjacent cavities is the distance that a particle travels during ½ period, or one period, depending on the phase shift of the resonant standing wave from one cavity to the next at that point along the beam line.
Early standing wave accelerators, i.e., those constructed before the development of side-cavity coupled accelerators in the 1960's, were either “0-mode” or “π mode” accelerators. The term “0-mode” has been most commonly used to mean that there is a 0° phase shift in the resonant RF signals from one cavity to the next. The term “π mode” has been most commonly used to mean that there is a 180° phase shift from cavity to cavity. These alternatives were used because other modes have a shunt impedance that is smaller by a factor of two, and high shunt impedance is a measure of the efficiency of an accelerator. This is because the amplitudes of the fields in the cavities have a sinusoidal distribution and all cells have the same phase, so only the cells at the maximum of the sinusoidal distribution are optimally phased for acceleration of the particles. The cavities near the nodes are approximately 90° out of phase from the particles.
In the traditional π/2 mode, there is a 90° phase shift from cavity to cavity, such that half of the cavities are unexcited and thus do not effect any acceleration. Nevertheless, the problem with a 0 or π mode structure is that the dispersion curve at both 0 and π has a slope of zero, so the mode separation is very small. With a small mode separation a significant amount of the input power is dissipated by exciting the modes adjacent to the desired mode, which do not contribute to the acceleration of the particles. Furthermore, excitation of undesired modes disturbs the desired electric field pattern and changes the way the particles absorb energy, and thus disturb the synchronism between the particle beam and the standing waves.
The π/2 mode is desirable because its dispersion curve, which describes the phase advance per cavity as a function of the operating frequency, is steepest and it has the largest inter-mode spacing for a structure of a particular size. However, a standing wave accelerator must be constructed with a larger number of cavities if adjacent cavities are coupled with a π/2 phase advance, because in such an arrangement every other cavity is a “dead,” or nonaccelerating, cavity. A primary solution to this problem has been to place the dead cavities off-axis, which results in what is known as a side-cavity coupled accelerator. While the dead off-axis cavities have little or no electromagnetic field, they nevertheless couple the on-axis accelerating cavities together. Side-cavity coupled accelerators, or any other cavity arrangement with a π/2 phase advance, have the advantage of a reduced sensitivity to construction tolerances. Also, such structures are phase-stabilized in the sense that RF losses do not bring about phase shifts between the accelerating cavities. Because of the symmetry associated with being in the middle of the pass band, the π/2 mode has significantly more relaxed tolerances than any other mode.
Most relatively recent proton accelerators have been constructed to include three stages positioned in sequence along the length of the accelerator: an initial radiofrequency quadrupole (RFQ) stage; a drift tube linac (DTL) stage; and a side-cavity coupled linac (SCL) stage. The transition from the DTL stage to the SCL stage is typically positioned at a point in the accelerator at which the velocity of the accelerated particles reaches a velocity of 0.4 to 0.5 times the speed of light, at which the shunt impedance of the DTL and SCL linac stages are almost equivalent.
The DTL technology was developed in the late 1940's and is still the most widely used technology at lower beam velocities. Although DTL-based proton acceleration linacs have been used for many years, they are relatively bulky and difficult to service. They require a different RF power source than the higher energy segments of the accelerator, and are expensive to fabricate because of the need to incorporate quadrupole magnet focusing cells within the drift tubes.
Accordingly, it is the object and purpose of the present invention to provide a linear particle accelerator having accelerator cavities that are simpler and less expensive to construct and service and, in particular, which are free of side cavities or other off-axis coupling cavities.
In particular, it is an object and purpose of the present invention to provide a simpler linear particle accelerator structure that achieves the foregoing objective and which is suitable for use in the high-energy range that has previously been addressed with side-cavity coupled structures, up to and including energy levels approaching the velocity of light, or one MeV and above in the case of electrons.
It is also an object to provide a simpler linear particle accelerator that may also be useful as well in the lower-energy range previously addressed with DTL technologies.